2014-03-03_REVISION - M1988112 - Laserfiche WebLink

Home Browse Search

4.2.3 Drop Structure Inlet Acceleration Chute The design of the inlet acceleration chute at the top of the drop structure appears to have neglected consideration of the amount of head required to accelerate the flow. Review of the design calculations and the as -built structure indicate that the existing structure may not be able to perform as originally intended under the design flood event (100 -year flood). Calculations completed during HEC -RAS modeling described in Section 4.3 conclude that the conveyance capacity of the structure will be less than calculated by the original design analysis. Without adequate head to accelerate the flow, and due to the abrupt transition from a rectangular channel into a circular conduit, under high flows it is anticipated that the inlet structure will be submerged, backing water up until it enters the conduit from a ponded state. During large runoff events, some of the ponded water is expected to bypass the inlet and sheet flow over the top of the drop structure and down the abutment slope. Because the inlet will be submerged, and the conduit will flow partially full downstream from the inlet, with no air vent provided, the flow will likely be unstable, causing pulsation and possibly cavitation within the conduit. No trash rack was provided, so there may be a tendency to plug the entrance to the conduit in high flows if significant trash is present in the flow. 4.2.4 Flip Bucket and Plunge Basin The outlet flip bucket structure appeared to be in good condition at the time of the inspection. The area surrounding the outlet structure had been maintained in February-March 2013 to control erosion including surface grading, placement of large riprap armoring in the plunge basin below the flip bucket, and construction of a rock check dam on the slope above the outlet. With the substantial rock armoring, it is difficult to evaluate the geometry of the plunge basin. Because no calculations of the flip bucket/plunge basin were available, supplemental calculations were prepared as part of this investigation and are presented in Section 4.3 subsequently. 4.3 RESULTS OF NEW EVALUATIONS PERFORMED AS PART OF DETAILED DAM SAFETY EVALUATION PROCESS 4.3.1 Hydrologic Check Analysis A separate analysis of the PMF was completed as a comparison to the values previously developed as part of the facility freeboard and capacity design. The new PMF was calculated - using an 8 -hour storm with a Probable Maximum Precipitation (PMP) of 14 inches, which is the same as that used for the original design evaluation. The original PMF evaluation generated a total runoff volume for the entire basin of 680 AF, whereas the new evaluation yielded 686 AF, essentially the same result. For the check analysis, hydrographs were developed using HydroCAD -10 Stormwater Modeling software using a standard SCS unit hydrograph with a True Peak Factor of 483. A SCS type 2 San Luis Project Miller Geotechnical Consultants Tailing Dam Data Report 27 February 2014